The topic is not explicitly discussed, as far as I know, in European or American epistemologists (who all seem to assume that it obviously is), whereas it is relevant in South Asian epistemology of language.
Graheli’s contribution to this workshop focuses on the epistemology of written versus spoken language in the Nyāya school of philosophy, since written language is a case in which the seeming transparency of language is revealed to be illusory. This revelation can take the form of the realisation that reading needs additional skills on top of the ones required to understand one’s mother tongue. In the case of Nyāya, the epistemic account of written words sees the knowledge conveyed through written words as involving an inference, from the written to the audible words. Thus, can written language still amount to a separate instrument of knowledge? If not, how can one avoid the risks of reductionism?